1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of the Related Art
A conventional motor drive circuit will be explained with reference to FIGS. 4, 5, and 6. FIG. 4 is a circuit block diagram showing a usual motor drive circuit. FIG. 5 is a circuit diagram showing a PWM circuit of a synchronous rectification type provided in the motor drive circuit of FIG. 4. FIG. 6 is a waveform diagram showing essential waveforms for FIG. 4. It is assumed that in the prior art, the motor drive circuit except the coil is an integrated circuit where bipolar transistors and CMOS transistors are mixed and integrated on the same chip. Furthermore, in the motor drive circuit, the source-side transistors and the sink-side transistors connected to the coil are supplied with control signals output from the PWM circuit so as to operate.
For an NPN-type bipolar transistor 2 (first source-side transistor) and an NPN-type bipolar transistor 4 (first sink-side transistor), the collector-emitter paths thereof are connected in series between a power supply VCC and ground VSS. Moreover, for an NPN-type bipolar transistor 6 (second source-side transistor) and an NPN-type bipolar transistor 8 (second sink-side transistor), the collector-emitter paths thereof are connected in series between the power supply VCC and ground VSS. Furthermore, a coil 14 is connected externally between a terminal 10 drawn out from the collector-emitter joint of the bipolar transistors 2, 4 and a terminal 12 drawn out from the collector-emitter joint of the bipolar transistors 6, 8. The bipolar transistors 2, 4, 6, 8 are turned on and off with control signals OUT1, OUT2, OUT3, and OUT4 supplied from the PWM circuit described later. That is, during the time period when the bipolar transistors 2, 8 are both on, a current in an R direction is supplied to the coil 14. In contrast, during the time period when the bipolar transistors 6, 4 are both on, a current in an L direction is supplied to the coil 14. And the bipolar transistors 2, 8 and the bipolar transistors 6, 4 operate complementarily, so that the current through the coil 14 changes in direction as required, and thereby the motor rotates in a predetermined direction.
A control circuit 16 outputs the control signals OUT1, OUT2, OUT3, and OUT4 for controlling on-off timing of the bipolar transistors 2, 4, 6, 8. The control circuit 16 has a PWM circuit 18 for rotating the motor at a predetermined speed. The PWM circuit 18 outputs the control signals OUT1, OUT2, OUT3, and OUT4 for making the bipolar transistors 2, 4, 6, 8 operate, based on input signals IN1, IN2 that change complementarily.
Because the motor drive circuit is a circuit where the bipolar transistors and CMOS transistors are mixed, the PWM circuit 18 is capable of outputting the control signals OUT1, OUT2, OUT3, and OUT4 for synchronously rectifying the bipolar transistors 2, 4, 6, 8. Specifically, when supplying a current in the R direction to the coil 14, the PWM circuit 18 outputs the control signals OUT1, OUT2, OUT3, and OUT4 for turning the bipolar transistor 6 off, the bipolar transistor 8 on, and complementarily the bipolar transistors 2, 4 on and off. In contrast, when supplying a current in the L direction to the coil 14, the PWM circuit 18 outputs the control signals OUT1, OUT2, OUT3, and OUT4 for turning the bipolar transistor 2 off, the bipolar transistor 4 on, and complementarily the bipolar transistors 6, 8 on and off. And, by setting the speed at which the bipolar transistors 2, 4 and the bipolar transistors 6, 8 are each complementarily turned on and off as required, the motor rotates at a predetermined speed.
The PWM circuit 18 is constituted by, for example, IIL (Integrated Injection Logic). Note that the IIL is circuit technology wherein inverters made up of bipolar transistors are connected as needed so that the signal at the intersection of signal lines is a logical product. The above related art is described in, for example, Japanese Patent Laid-open Publication No. 2002-272162.
When the bipolar transistors 2, 4 and the bipolar transistors 6, 8 are each complementarily turned on and off, due to the characteristic of the coil 14, currents occurring between the power supply VCC and ground VSS pass through along the collector-emitter paths of the bipolar transistors 2, 4 and the bipolar transistors 6, 8, and thus the motor drive circuit may malfunction or be destroyed.
Hence, the PWM circuit 18 outputs the control signals OUT1, OUT2 having time periods TAA when the bipolar transistors 2, 4 are both off in a time period TA when the bipolar transistors 2, 4 are complementarily turned on and off, and in contrast, outputs the control signals OUT3, OUT4 having time periods TBB when the bipolar transistors 6, 8 are both off in a time period TB when the bipolar transistors 6, 8 are complementarily turned on and off. By this means, the impedances of terminals 10, 12, to which the coil 14 is connected, become infinite (Z) during time periods TAA and TBB, and thus currents do not pass through along the collector-emitter paths of the bipolar transistors 2, 4 and the bipolar transistors 6, 8.
However, a plurality of time periods TAA exist in time period TA and also a plurality of time periods TBB exist in time period TB, and hence, as the speed at which the bipolar transistors 2, 4 and the bipolar transistors 6, 8 are each complementarily turned on and off becomes higher, it becomes harder to ignore time periods TAA and TBB. Therefore, there is the problem that the conventional motor drive circuit is unsuitable for the specification that the bipolar transistors 2, 4 and the bipolar transistors 6, 8 are each complementarily turned on and off at high speed (PWM-driven at high speed).